Effects of photoperiods on PK2 mRNA rhythm in the SCN. Temporal profiles of PK2 rhythm under 8L:16D (A), 16L:8D (B) and 20L:4D (C). Open and filled bars indicate light and dark periods, respectively. The zeitgeber time (ZT) on the x-axis reflects the timescale for each photoperiod. Each value represents the mean ± SEM of 3–4 animals. One-way ANOVA indicated that peak levels of all groups are significantly different from each other, p < 0.05–0.001 using Bonferroni's post-hoc test. Autoradiographic images show representative mRNA expression of PK2 during these photoperiods.